since you normally have fairly low flow through GFO, it seems a perfect application for the Ca reactor effluent.
About salt: You cannot put all your eggs in one testor or test. Batches change. I recently tested gettanked and Red Sea Pro and both had zero PO4 based on my results. In fact the gettanked was virtually polutant free, at least from the perspective of the tests I can currently do.
And the only blip for the Red Sea Pro was a small amount of silicates. This can be problematic in terms of fueling algal growth, but I would still consider it usable. Also note that both of these salts had zero ammonia, appropriate Ca and alk levels, zero trates, appropriate Mg levels, etc.
Although I have never tested R C myself, I have never seen a completely positive report. I do know that some reef keepers use it in a two salt combination.
Sediment filters are unlikely to help in the case of PO4, with the exception that they will allow subsequent cartridges to perform better. I have heard apposing arguments on the size of pre-filters and I suggest you talk to John at SpectraPure about that. You might also get his opinion on AWT.
Again, he will be able to recommend the appropriate resin(s) for your situation. I used to believe that resin was all coming from the same place, and bargain shopped on eBay for it. I have suspended that practice as I noticed a dramatic decrease in resin performance over the course of a year. I now only buy resin from SpectraPure. Why? Because I know they wouldn't screw their reputation by selling an inferior product. Yes it does cost more, but in the long run, it performs better and with greater consitency. Peace of mind is worth a few extra bucks IMO.
And a word of advice: You say the "next time you mix up water for a water change..." I would propose that you change your method slightly. When I do a water change, I immediately mix the next batch. With systems the size of ours, I believe having ready made water is extremely important. When something goes wrong, do you panic and use tap water? How many stories have we read about how bad that can go?
I always have about 90g ready made, with another 90g of purified water standing by. Compared to my water volume it's not a lot, but at least it's something. It certainly is enough to fill an emergency tank should things go radically wrong. And you want to make darn sure that water has zero silicates. Often we react to algal growth by doing water changes, but if your new water has silicates, you are actually giving more fuel to the fire.
Regarding the Gen-X media: I am a fan, but pre-soaking can really help. And I am running plain SW through it for much of my 24 hour cycle due to pH fluctuations. At night, there is no CO2 being injected. BUT, what I do find is that it readily collects detritus. So when I swap carbon or add Ca media, I always remove the "older" media and give it a thorough rinsing in RO/DI water. A lot of crap comes out. Again another reason for keeping a lot of RO/eDI water on hand. I have a hose & garden sprayer attached to my elevated RO/DI tank for just such things. I also use it to rinse test tubs and all equipment I service.
Sorry for the long-winded post.
No ride in sight yet.
